Mixed-typed heterojunction thin-film solar cell structure and method for fabricating the same

ABSTRACT

The present invention discloses a mixed-type heterojunction thin-film solar cell structure and a method for fabricating the same. Firstly, a conductive substrate and a template are provided, and the template has a substrate and an inorganic wire array formed on the substrate. Next, a conjugate polymer layer is formed on the conductive substrate. Next, the inorganic wire array is embedded into the conjugate polymer layer. Next, the substrate is separated from the inorganic wire array. Then, an electrode layer is formed over the inorganic wire array and the conjugate polymer layer. The solar cell structure of the present invention has advantages of flexibility, high energy conversion efficiency and low fabrication cost.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating a solar cellstructure, particularly to a mixed-type heterojunction thin-film solarcell structure and a method for fabricating the same.

2. Description of the Related Art

Fossil fuels are going to be exhausted by the end of this century. Thesearch and development for substitute energies, such as wind power, tidepower, and biofuel, has been progressing for a period of time. Amongthem, solar energy has relative higher market acceptability, and manynations have been devoted to the development of solar energy. The GermanAdvisory Council on Global Change predicted that solar energy willprovide about 60% of the total energy in 2100. Solar energy is generatedby the photovoltaic effect, wherein solar energy material directlyconverts sunlight into electric energy. The crystalline silicon solarcell has been developed for tens of years, and the related technologiesthereof have been very mature. Generally, the monocrystalline siliconsolar cell has an energy conversion efficiency of as high as about 20%.However, the fabrication cost thereof is too high to popularize solarenergy. The topics of solar energy researches would be developing newmaterials, processes and systems to promote the energy conversionefficiency and reduce the cost of solar energy.

As to the polymer solar cell, the bi-layer solar energy elementcontaining donors and acceptors was the one used at first. However, thecontact area between donors and acceptors is too small to increase theprobability of exciton (hole-electron pair) fission. Thus, theperformance of this type of polymer solar cell is hard to promote. Thesolar cell containing the mixed donor material and acceptor material iscalled the BHJ (bulk heterojunction) cell, which has a greaterdonor-acceptor contact area than the bi-layer solar cell. Further, thespacing between donors and acceptors is within nanometers, which is nearthe exciton diffusion length—about 1-10 nm. Thus, the probability ofexciton fission increases. The local electric field in the junction,which disjoins charges, originates from the HOMO (Highest OccupiedMolecular Orbital) of donors and LUMO (Lowest Unoccupied MolecularOrbital) of acceptors. Therefore, more electrons and holes are disjoinedat junctions. Thus is solved the problem of exciton diffusion length inpolymer semiconductor. Then, the entire active layer is able to convertlight into current. The mixture of donor material and acceptor materialis spin-coated on a substrate to form an active layer. However, the twomaterials are not uniformly distributed in the active layer, whichresults in the following two cases. One is that there are likely to bepaths directly connecting the anode and cathode, decreasing the parallelresistance and increasing leakage current. The other is that there arelikely to be isolated areas having none path to the electrodes,wherefore the external circuit cannot collect the current of holes andelectrons separated in the isolated areas.

Accordingly, the present invention proposes a mixed-type heterojunctionthin-film solar cell structure and a method for fabricating the same tosolve the abovementioned problems, whereby solar cells can possessflexibility and high energy conversion efficiency at the same time.

SUMMARY OF THE INVENTION

The primary objective of the present invention is to provide amixed-type heterojunction thin-film solar cell structure and a methodfor fabricating the same, whereby solar cells possesses flexibility andhigh energy conversion efficiency.

Another objective of the present invention is to provide a mixed-typeheterojunction thin-film solar cell structure and a method forfabricating the same, which can reduce the cost of solar cells.

To achieve the abovementioned objectives, the present invention proposesa mixed-type heterojunction thin-film solar cell structure and a methodfor fabricating the same. The method of the present invention comprisessteps: providing a conductive substrate and a template, wherein thetemplate has a substrate and an inorganic wire array formed on thesubstrate; forming a conjugate polymer layer on the conductivesubstrate; embedding the inorganic wire array into the conjugate polymerlayer; separating the substrate from the inorganic wire array; andforming an electrode layer over the inorganic wire array and theconjugate polymer layer.

Below, the embodiments are described in detail in cooperation with thedrawings to make easily understood the technical contents,characteristics and efficacies of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing a solar cell structureaccording to one embodiment of the present invention;

FIGS. 2( a)-2(g) are diagrams schematically showing the steps offabricating a solar cell structure according to the present invention;

FIG. 3 is a diagram schematically showing a solar cell structureaccording to another embodiment of the present invention;

FIGS. 4( a)-4(g) are diagrams schematically showing the steps offabricating another solar cell structure according to the presentinvention;

FIG. 5 is a diagram schematically showing a solar cell structureaccording to a further embodiment of the present invention;

FIGS. 6( a)-6(h) are diagrams schematically showing the steps offabricating a further solar cell structure according to the presentinvention;

FIGS. 7( a)-7(d) are diagrams schematically showing an etching methodfor fabricating silicon nanowires according to the present invention;

FIG. 8 is a diagram schematically showing the method of separating thechip of a silicon nanowire array from a conjugate polymer layeraccording to the present invention;

FIGS. 9( a)-9(b) are diagrams schematically showing that aselectively-etched layer is formed in between an inorganic wire arrayand a substrate thereof according to the present invention; and

FIG. 10 is a diagram schematically showing that an inorganic wire arraycontains nanowire structures according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention proposes a solar cell structure, which integratesa conjugate polymer and an inorganic semiconductor nanowire. Conjugatepolymer is very suitable for the process of fabricating a large-area,low-cost and flexible solar cell. However, the solar cell containingonly conjugate polymer is hard to achieve a high efficiency becauseconjugate polymer has a low carrier mobility and a narrow sunlightabsorption spectrum. Contrarily, inorganic semiconductor has a highercarrier mobility and a higher sunlight absorption ability. For example,a gallium arsenide nanowire array can offset the portion of sunlightabsorption spectrum, which the P3HT (poly-3-hexylthiophene) conjugatepolymer does not have. Therefore, the solar cell structure of thepresent invention can possess flexibility and a high energy conversionefficiency simultaneously.

Refer to FIG. 1 for a solar cell structure according to the presentinvention. The solar cell structure of the present invention comprises aconductive substrate 10, a hole transport layer 12 formed on theconductive substrate 10, a conjugate polymer layer 14 formed on the holetransport layer 12, an inorganic wire array 16, such as a nanowirearray, inserted into the conjugate polymer layer 14 but not contactingthe conductive substrate 10, a hole blocking layer 22 formed on theconjugate polymer layer 14 and covering the conjugate polymer layer 14and the inorganic wire array 16, and an electrode layer 24 formed on thehole blocking layer 22 and used to provide electricity. Alternatively,an electron-blocking layer is used to replace the hole transport layer12. Similarly, the electron-blocking layer does not contact theinorganic wire array 16.

In addition to the abovementioned structure, the hole blocking layer 22may be omitted, and only the electrode layer 24 covers the conjugatepolymer layer 14 and the inorganic wire array 16 in the solar cellstructure of the present invention.

The inorganic wire array 16 is made of a single element, a binarycompound semiconductor or a compound semiconductor containing more thantwo components. The conjugate polymer layer 14 is made of a materialselected from a group consisting of P3HT (poly-3-hexylthiophene), MEHPPV(poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]), PCPDTBT(poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]),OC₁C₁₀-PPV (Poly[2-(3,7-dimethyloctyloxy)-5-methoxy-p-phenylenevinylene]), MDMO-PPV(poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene]), andpolyfluorene. Among conjugate polymer materials, P3HT has a higher holemobility. In a PTFT (Polymer Thin-Film Transistor) research, P3HT has ahole mobility of as high as 0.1 cm2/V-sec. General conjugate polymermaterials have a hole mobility of only 10⁻¹-10⁻⁷cm2/V-sec. A lowelectron mobility will cause electrons to deposit in the active layerand reduce the efficiency of solar cells. The conductive substrate 10 ismade of one of the following materials: a transparent conductivesubstrate, a transparent-electrode glass substrate, atransparent-electrode plastic substrate, a transparent-electrode quartzsubstrate and a thin metallic plate. The electrode layer 24 is made of ametal or a transparent-electrode material, such as a material selectedfrom a group consisting of ITO (Indium Tin Oxide), GITO (Gallium IndiumTin Oxide), ZITO (Zinc Indium Tin Oxide), FTO (Fluorine-doped TinOxide), ZnO (Zinc Oxide), AZO (Aluminum Zinc Oxide) and IZO (Indium ZincOxide). The hole blocking layer 22 is made of a material selected from agroup consisting of ZnO (Zinc Oxide), TiO_(x) (titanium oxide), PCBM((6,6)-phenyl C₆₁ butyric acid methyl ester), LiF (Lithium Fluoride), acalcium compound and an alkali compound, wherein the alkali compoundsinclude Li₂O, LiBO₂, K₂SiO₃, and Cs₂CO₃. The hole transport layer 12 ismade of a material selected from a group consisting of PEDOT(Poly(3,4-ethylenedioxythiophene)), PEDOT:PSS(Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)), TFB:TPDSi₂(poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine]:4,4′-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl), CuPc(copper phthalocyanine), and TNATA(4,4′,4″-tris-N-naphthyl-N-phenylamino-triphenylamine). Theelectron-blocking layer is made of a material selected from a groupconsisting of TPD(N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine), BFE(poly(9,9-dioctylfluorene-co-N,N′-di(phenyl)-N,N′-di(3-carboethoxyphenyl)benzidine), NPB(4,4-bis[N-(1-naphthyl-1-)-N-phenyl-amino]-biphenyl), TPTE(N,N′-diphenyl-N,N′-bis(di(3-methylphenyl)aminobiphenyl)benzidine), apolycarboxy-polymer, a quaternized polyamine-polymer, apolysulphato-polymer, a polysulpho-polymer, and a poly (vinylphosphonicacid). The conjugate polymer layer 14 has a thickness of between 30 nmand 5 μm. Each wire of the inorganic wire array 16 has a length ofbetween 50 nm and 50 μm. The spacing between two wires is below 50 timesthe width of the wire. The depth by which the wire of the inorganic wirearray 16 is inserted into the conjugate polymer layer 14 is between 30nm and 5 μm.

Refer to from FIG. 2( a) to FIG. 2 (g) diagrams schematically showingthe steps of fabricating the solar cell structure of the presentinvention. As shown in FIG. 2( a), a conductive substrate 10 is providedfirstly. Next, as shown in FIG. 2( b), a hole transport layer 12 isformed on the conductive substrate 10. Next, as shown in FIG. 2( c), aconjugate polymer layer 14 is formed on the hole transport layer 12; theconjugate polymer layer 14 is formed with one of the following methods:spin coating, dip coating, inkjet printing, contact printing, screenprinting, evaporation, sputtering, parylene coating, and electrochemicaldeposition. Next, as shown in FIG. 2( d), a template 18 is provided, andthe template 18 contains a semiconductor substrate 20 and an inorganicwire array 16 formed on the semiconductor substrate 20; the inorganicwire array 16 is embedded into the conjugate polymer layer 14. Next, asshown in FIG. 2( e), the semiconductor substrate 20 is separated fromthe inorganic wire array 16. The inorganic wire array 16 is embeddedinto the conjugate polymer layer 14 to such a depth that the wires donot contact the hole transport layer 12 or the conductive substrate 10.The spacing of two wires is not limited to but had better be below twotimes the diffusion length of excitons. The abovementioned spacing is tofacilitate exciton fission and reduce the probability of recombinationof electrons and holes after the conjugate polymer among the inorganicwire array 16 has absorbed sunlight. The length of the wires of theinorganic wire array 16 should be adjusted according to the absorptioncoefficient of the inorganic material. In this embodiment, the wire ofthe inorganic wire array 16 is a nanowire. Next, as shown in FIG. 2( f),a hole blocking layer 22 is formed over the conjugate polymer layer 14and the inorganic wire array 16 via a spin coating method or a vapordeposition method. The thickness of the hole blocking layer 22 should belimited to below a specified value lest the series resistance of thesolar cell be increased and the efficiency be decreased. Next, as shownin FIG. 2( g), an electrode layer 24 is formed on the hole blockinglayer 22 via screen printing, vapor deposition, sputtering, or applyingsilver glue. Thus is completed a solar cell. Alternatively, anelectron-blocking layer is used to replace the hole transport layer 12.Similarly, the inorganic wire array 16 does not contact theelectron-blocking layer.

The hole blocking layer 22 may be omitted from the abovementioned solarcell structure, and only the electrode layer 24 directly covers theconjugate polymer layer 14 and the inorganic wire array 16. Such astructure can be achieved via cancelling the step shown in FIG. 2( f).

Refer to FIG. 1 and FIG. 3. FIG. 3 shows another solar cell structure ofthe present invention. The structure of FIG. 3 is different from thestructure of FIG. 1 in that a non-conductive layer 26 is formed on theconjugate polymer layer 14 to protect the inorganic wire array 16. Thenon-conductive layer 26 has such a thickness that the inorganic wirearray 16 emerging from the conjugate polymer layer 14 is higher than thenon-conductive layer 26. Then, the electrode layer 24, which is used toprovide electricity, is formed over the non-conductive layer 26 and theinorganic Wire array 16.

The non-conductive layer 26 is made of a transparent material or anopaque material. The transparent material may be one of the followingmaterials: PMMA (polymethylmethacrylate), SOG (Spin-On Glass), SiO₂(silicon dioxide), and Si₃N₄ (silicon nitride).

Refer to from FIG. 4( a) to FIG. 4( g) diagrams schematically showingthe steps of fabricating the solar cell structure shown in FIG. 3. Thesteps shown in from FIG. 4( a) to FIG. 4( e) is identical to the stepsshown in from FIG. 2( a) to FIG. 2( e), which have been describedthereinbefore and will not repeat here. After the structure shown inFIG. 4( e) is completed, a non-conductive layer 26 is formed on theconjugate polymer layer 14 to protect the inorganic wire array 16, asshown in FIG. 4( f); the non-conductive layer 26 may be formed via oneof the following methods: spin-coating, vapor deposition, sputtering,and dip coating; then, the non-conductive layer 26 is etched to havesuch a thickness that the inorganic wire array 16 emerges from thenon-conductive layer 26, and the etching is undertaken with a dryetching method or a wet etching method. Next, as shown in FIG. 4( g), anelectrode layer 24 is formed over the non-conductive layer 26 and theinorganic wire array 16. Thus is completed a solar cell structure.

Refer to FIG. 3 and FIG. 5. FIG. 5 shows a further solar cell structureof the present invention. The structure of FIG. 5 is different from thestructure of FIG. 3 in that a hole blocking layer 22 is formed on thenon-conductive layer 26. Then, an electrode layer 24, which is used toprovide electricity, is formed over the hole blocking layer 22.

Refer to from FIG. 6( a) to FIG. 6( h) diagrams schematically showingthe steps of fabricating the solar cell structure shown in FIG. 5. Thesteps shown in from FIG. 6( a) to FIG. 6( e) is identical to the stepsshown in from FIG. 2( a) to FIG. 2( e), which have been describedthereinbefore and will not repeat here. After the structure shown inFIG. 6( e) is completed, a non-conductive layer 26 is formed on theconjugate polymer layer 14 to protect the inorganic wire array 16, asshown in FIG. 6( f); then, the non-conductive layer 26 is etched to havesuch a thickness that the inorganic wire array 16 emerges from thenon-conductive layer 26. Next, as shown in FIG. 6( g), a hole blockinglayer 22 is formed on the non-conductive layer 26. Next, as shown inFIG. 6( h), an electrode layer 24 is formed on the hole blocking layer22. Thus is completed a solar cell structure.

Below is described the process of fabricating an inorganic semiconductornanowire array. Refer to from FIG. 7( a) to FIG. 7( d), wherein theprocess of electrochemically fabricating silicon nanowires with a wetetching method is used as an exemplification. The etching solution is amixture of sliver nitrate solution and hydrofluoric acid. As shown inFIG. 7( a), a silicon substrate 28 is provided firstly. Next, as shownin FIG. 7( b), the silver nitrate solution electrolessly deposits silverions 30 on the silicon substrate 28. Next, as shown in FIG. 7( c), thehydrofluoric acid etches the areas where silver ion 30 deposits. Theresult of etching is shown in FIG. 7( d). Thus is attained a siliconnanowire array.

In the abovementioned method, a crystalline substrate/epitaxialstructure having the composition of intended nanowires is used tofabricate nanowires. Therefore, the nanowires also haves the superiorquality of the crystalline substrate/epitaxial structure. Further, theoriginal crystalline substrate can be reused to fabricate nanowires,whereby the very expensive crystalline substrate can be fully used, andthe fabrication cost is greatly reduced.

Below is described in detail the process of fabricating siliconnanowires. Firstly, a silicon substrate is cleaned, wherein the siliconsubstrate is sequentially placed in ultrasonic vibrators respectivelycontaining acetone, methanol, and deionized water, and the cleaning timefor each ultrasonic vibrator is 5 minutes. Next, the silicon substrateis blow-dried with a nitrogen injector. Next, 0.2 grams of silvernitrate, 12 ml of hydrofluoric acid, and 40 ml of deionized water aresequentially poured into a beaker. Next, the cleaned silicon substrateis placed in the beaker and etched therein for 20 minutes. Next, thesilicon substrate is taken out from the beaker, cleaned with deionizedwater, and then blow-dried with a nitrogen injector. Next, the siliconsubstrate is placed in a solution, which has nitric acid and deionizedwater of a ratio of 1:1, to remove the silver dendrite structure on thesurface of the silicon substrate, and then the silicon substrate istaken out immediately. Next, the silicon substrate is cleaned withdeionized water and blow-dried with a nitrogen injector. Next, thesilicon substrate is placed in a dilute hydrofluoric solution (BOE,Buffered Oxide Etch) for 30 seconds and then taken out from thesolution. Next, the silicon substrate is cleaned with deionized waterand blow-dried with a nitrogen injector. Thus is completed a siliconnanowire array.

In addition to the abovementioned method, the inorganic semiconductornanowire array may be fabrication with other methods, such as a wetetching method and a dry etching method. The wet etching method may beone of the following methods: chemical solution etching andphoto-enhanced electrochemical etching. The dry etching method may beone of the following methods: RIE (Reactive Ion Etching), HDP(high-density plasma) etching, plasma etching, sputtering etching, andreactive ion beam etching. Herein, two methods to fabricate the masks ofdry etching are to be introduced. One is spin-coating a silicon-dioxidenanoparticle colloidal suspension to form a monolayer of silicon dioxidenanoparticles on an inorganic substrate, wherein the viscosity of thesuspension should be modified before spin-coating. The other is coatinga very thin metal film on an inorganic semiconductor substrate and fastannealing the metal film to form a dry etching mask with a nanometricisland pattern.

There are also other methods to fabricate ordered semiconductornanowires, including the chemical vapor deposition method,molecular-beam epitaxy method, AAO (Anodic Aluminum Oxide) method,electrochemical method, hydrothermal method, and VLS(Vapor-Liquid-Solid) method. In the hydrothermal method, water is usedas the medium; reactants and water are enclosed in a reactor and thenheated and pressurized to undertake reaction; after reaction, water isfiltered out to obtain the products, and then the products are cleanedand dried. Some materials sensitive to water or unstable in water areunsuitable to react in a water solution. In such a case, an organicsolvent may be used as the medium, which is known as the solvothermalmethod. Using an organic solvent as the medium greatly expands theapplication field of the hydrothermal method. Sometimes, two organicsolvents are used at the same time to change the polarity of the mediumin the solvothermal method.

When a zinc oxide nanowire array is fabricated with the hydrothermalmethod, a seed layer is used as the nucleation sites, and water is usedas the medium; then, the reactants are enclosed, heated and pressurizedin a reactor to react and grow nanowire arrays. At present, there havebeen many researches about the hydrothermal method, including the size,length, and density of nanowire arrays. The hydrothermal method has thefollowing advantages: firstly, any substrate, which zinc oxide can bespin-coated on to form a film, can be used to fabricate zinc oxidenanowires; secondly, an environment of a low temperature and theatmospheric pressure is sufficient to grow the zinc oxide nanowires.However, how to find a suitable precursor for a given nanowire is alwaysa tough problem in the hydrothermal method. Therefore, the hydrothermalmethod can fabricate only few types of nanowires at present.

The VLS method proposed by Wager and Ellis in 1964 is a common method togrow III-V group or semiconductor nanowires. In the VSL method, ametallic catalyst is used as a medium to deliver vapor-phase atoms. Theatoms diffuse through the liquid metal to the bottom substrate where theatoms stack to form nanowires. In the VLS method, a specified materialhas to be grown on a specified substrate (usually a substrate made of asimilar material) lest lattice mismatch occur. In 2007, Stelzner et al.grew silicon nanowires on a silicon substrate via different metals, suchas gallium, indium, aluminum and gold. In 2005, Mohan et al. used ane-beam lithography technology to grow an indium-phosphide nanowire on anindium-phosphide substrate. In 2006, Tutuc et al. coated gold on asilicon substrate and used germanium tetrahydride gas to grow germaniumnanowires. In 2006, Morber et al. adopted gold as the catalyst to growiron oxide nanowires on an aluminum oxide substrate. In 2006, Wan Qinghad grown very beautiful vertical ITO (Indium Tin Oxide) nanowires on anYSZ (Yttrium-Stabilized Zirconia) substrate coated with gold. The VLSmethod is still studied by many researchers and extensively used tostudy the properties of various types of nanowires. However, theequipment thereof is very expensive and hard to mass-fabricatenanowires.

Below is to be described the process of implanting an inorganicsemiconductor nanowire array into a conjugate polymer layer. Firstly,the conjugate polymer layer is heated to the Tg temperature (glasstransition temperature) thereof and maintained at the temperature for aperiod of time. Then, the substrate having the inorganic semiconductornanowire array is pressed onto the conjugate polymer layer at thetemperature, and the state is maintained for a period of time. The timeand temperature are varied with the requirements of experiments. Next,the sample is placed at the ambient temperature for a period of time tolet the conjugate polymer layer cool down, and the time is varied withthe requirements of experiments. Then, the substrate is separated fromthe inorganic semiconductor nanowire array. Thus, the inorganicsemiconductor nanowire array is embedded into the conjugate polymerlayer. The substrate is separated from the inorganic semiconductornanowire array via ultrasonic vibration, slight knockings, chemicaletching, or even directly lifting off the substrate. When an inorganicsemiconductor nanowire array is implanted into a conjugate polymerlayer, the following factors should be considered: the pressure appliedonto the inorganic semiconductor nanowire array, the uniformity of thepressure, the temperature of the conjugate polymer layer, the material,length, width, spacing and distribution of the nanowires. When thepressure is too small, the nanowires are hard to be inserted into theconjugate polymer layer. When the pressure is not uniform, the nanowirescannot be completely implanted into the conjugate polymer layer. Theconjugate polymer layer should be heated to a temperature higher thanthe Tg temperature thereof. The higher the temperature is, the easierthe implantation of the nanowires is.

Below is described in detail the process of implanting an inorganicsemiconductor nanowire array into a conjugate polymer layer. Refer toFIG. 8. In this embodiment, the conjugate polymer layer 14 is made ofP3HT, and the conductive substrate 10 is made of an ITO glass, and theinorganic wire array 16 is exemplified by a silicon nanowire array.Firstly, the ITO glass is cleaned, wherein the ITO glass is sequentiallyplaced in ultrasonic vibrators respectively containing acetone, methanoland deionized water, and the cleaning time for each ultrasonic vibratoris 5 minutes. Next, the ITO glass is blow-dried with a nitrogeninjector. Next, 0.08 grams of P3HT is dissolved in 3 ml ofdichlorbenzene, and the solution is spin-coated on the ITO glass for 60seconds at a rotation speed of 600 rpm. Next, the coated ITO glass isplaced at the ambient temperature for 30 minutes. Next, the coated ITOglass is annealed at a temperature of 160° C. for 5 minutes with ahigh-precision bidirectional-alignment vacuum hot-pressing machine.Next, a silicon nanowire chip 36 is pressed onto P3HT, and a plastic padis placed over the silicon nanowire array 36. Next, the plastic pad ispressurized by a pressure of 13 kg/cm² for about 10 minutes with thehigh-precision bidirectional-alignment vacuum hot-pressing machine, andthe upper and lower heating plates of the machine are maintained at atemperature of 160° C. Next, the sample is placed at the ambienttemperature for 30 minutes. Next, a hammer 32 is used to knock an objectglass 34 to separate the substrate of the silicon nanowire chip 36 fromP3HT. Thus, the silicon nanowire array is implanted from the siliconnanowire chip 36 to the P3HT layer.

When the inorganic wire array is a nanometric structure, the inorganicwire array has a total width of between 300 nm and 100 m on the originalsemiconductor substrate; each wire of the inorganic wire array has alength of between 50 nm and 50 μm; the section, which is vertical to theoriginal semiconductor substrate, of each wire has a width of between 5nm and 300 nm.

In addition to being a nanometric structure, the inorganic wire arraymay also be a micron structure or a submicron structure. The inorganicwire may be made of one of the following materials: silicon, germanium,gallium arsenide, indium phosphide, gallium phosphide, antimonyselenide, gallium antimonide, zinc telluride, and indium galliumnitride. Such a type of inorganic wire array has a total width ofbetween 300 nm and 100 m on the original semiconductor substrate; eachwire of the inorganic wire array has a length of between 50 nm and 50μm; the spacing between two wires is below 100 μm; When the inorganicwire array is a micron structure, the section, which is vertical to theoriginal semiconductor substrate, of each wire has a width of between300 nm and 3000 μm.

Refer to FIG. 9( a). When the nanostructure of a nanometric, micron orsubmicron inorganic wire array 40 has a lager section, or when thenanowires, micronwires, submicronwires or nanocolumns of the inorganicwire array 40 are very tough, ultrasonic vibration or knocking is hardto separate the inorganic wire array 40 from the original semiconductorsubstrate 38. In such a case, a selectively-etched layer 42 may beformed in between the wires and the original semiconductor substrate 38.The selectively-etched layer 42 will be completely etched away with adry or wet etching method. Thus, the inorganic wire array 40 can beseparated from the original semiconductor substrate 38 without seriouslydamaging the wire structure and the original semiconductor substrate 38.

Refer to FIG. 9( b). Alternatively, the selectively-etched layer 42 ispartially etched away after the step of fabricating the micron orsubmicron structure and the nanowires of the inorganic wire array 40 andbefore the succeeding steps. Thus, the nanowires, the nanocolumns, themicron structure or the submicron structure can be easily implanted toanother substrate.

Refer to FIG. 10. If the contact area between the conjugate polymerlayer and the micron or submicron structure of the inorganic wire array40 is smaller, additional nanowire structures 44 may be formed on themicron or submicron structure to increase the contact area and promotethe efficiency of collecting carriers.

Therefore, the solar cell structure of the present invention hasflexibility, high energy conversion efficiency and a low price at thesame time.

The embodiments described above are only to exemplify the presentinvention but not to limit the scope of the present invention.Therefore, any equivalent modification or variation according to thespirit of the present invention is to be also included within the scopeof the present invention.

1. A mixed-type heterojunction thin-film solar cell structure comprisinga conductive substrate; a conjugate polymer layer formed on saidconductive substrate; an inorganic wire array inserted into saidconjugate polymer layer but not contacting said conductive layer; and anelectrode layer covering said inorganic wire array and said conjugatepolymer layer.
 2. The mixed-type heterojunction thin-film solar cellstructure according to claim 1, wherein a hole blocking layer is formedin between said conjugate polymer layer and said electrode layer tocover said conjugate polymer layer and said inorganic wire array, saidhole blocking layer is made of a material selected from a groupconsisting of ZnO (Zinc Oxide), TiO_(x) (titanium oxide), PCBM((6,6)-phenyl C₆₁ butyric acid methyl ester), LiF (Lithium Fluoride), acalcium compound and an alkali compound, wherein said alkali compound isLi₂O, LiBO₂, K₂SiO₃, or Cs₂CO₃.
 3. The mixed-type heterojunctionthin-film solar cell structure according to claim 1, wherein anon-conductive layer is formed in between said conjugate polymer layerand said electrode layer to protect said inorganic wire array; saidnon-conductive layer has such a thickness that said inorganic wire arrayemerging from said conjugate polymer layer is higher than saidnon-conductive layer; said non-conductive layer is made of a transparentmaterial or an opaque material; said transparent material is one offollowing materials: PMMA (polymethylmethacrylate), SOG (Spin-On Glass),SiO₂ (silicon dioxide), and Si₃N₄ (silicon nitride).
 4. The mixed-typeheterojunction thin-film solar cell structure according to claim 3,wherein a hole blocking layer is formed in between said non-conductivelayer and said electrode layer to cover said non-conductive layer andsaid inorganic wire array, said hole blocking layer is made of amaterial selected from a group consisting of ZnO (Zinc Oxide), TiO_(x)(titanium oxide), PCBM ((6,6)-phenyl C₆₁ butyric acid methyl ester), LiF(Lithium Fluoride), a calcium compound and an alkali compound, whereinsaid alkali compound is Li₂O, LiBO₂, K₂SiO₃, or Cs₂CO₃.
 5. Themixed-type heterojunction thin-film solar cell structure according toclaim 1, wherein spacing between two wires of said inorganic wire arrayis below two times diffusion length of excitons in polymer.
 6. Themixed-type heterojunction thin-film solar cell structure according toclaim 1, wherein length of wires of said inorganic wire array isadjusted according to an absorption coefficient of an inorganic materialof said inorganic wire array.
 7. The mixed-type heterojunction thin-filmsolar cell structure according to claim 1, wherein at least one holetransport layer or at least one electron blocking layer is formed inbetween said conductive substrate and said conjugate polymer; either ofsaid hole transport layer and said electron blocking layer does notcontact said inorganic wire array.
 8. The mixed-type heterojunctionthin-film solar cell structure according to claim 1, wherein saidinorganic wire array is a nanometric structure, a micron structure, or asubmicron structure.
 9. The mixed-type heterojunction thin-film solarcell structure according to claim 8, wherein when said inorganic wirearray is a micron structure or a submicron structure, nanowirestructures are formed on said micron structure or said submicronstructure to increase contact area between said conjugate polymer layerand said inorganic wire array.
 10. The mixed-type heterojunctionthin-film solar cell structure according to claim 1, wherein saidinorganic wire array is made of a material selected from a groupconsisting of silicon, germanium, gallium arsenide, indium phosphide,gallium phosphide, gallium antimonide, zinc telluride, indium galliumnitride, a single element, a binary compound semiconductor, and acompound semiconductor containing more than two components; saidconjugate polymer layer is made of a material selected from a groupconsisting of P3HT (poly-3-hexylthiophene), MEHPPV(poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]), PCPDTBT(poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]),OC₁C₁₀-PPV (Poly[2-(3,7-dimethyloctyloxy)-5-methoxy-p-phenylenevinylene]), MDMO-PPV(poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene]), andpolyfluorene; said conductive substrate is made of one of followingmaterials: a transparent conductive substrate, a transparent-electrodeglass substrate, a transparent-electrode plastic substrate, atransparent-electrode quartz substrate and a thin metallic plate; saidelectrode layer is made of a metal, a transparent-electrode material, ora material selected from a group consisting of ITO (Indium Tin Oxide),GITO (Gallium Indium Tin Oxide), ZITO (Zinc Indium Tin Oxide), FTO(Fluorine-doped Tin Oxide), ZnO (Zinc Oxide), AZO (Aluminum Zinc Oxide)and IZO (Indium Zinc Oxide).
 11. The mixed-type heterojunction thin-filmsolar cell structure according to claim 7, wherein said hole transportlayer is made of a material selected from a group consisting of PEDOT(Poly(3,4-ethylenedioxythiophene)), PEDOT:PSS(Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)), TFB:TPDSi₂(poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine]:4,4′-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl), CuPc(copper phthalocyanine), and TNATA(4,4′,4″-tris-N-naphthyl-N-phenylamino-triphenylamine); saidelectron-blocking layer is made of a material selected from a groupconsisting of TPD(N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine), BFE(poly(9,9-dioctylfluorene-co-N,N′-di(phenyl)-N,N′-di(3-carboethoxyphenyl)benzidine), NPB(4,4-bis[N-(1-naphthyl-1-)-N-phenyl-amino]-biphenyl), TPTE(N,N′-diphenyl-N,N′-bis(di(3-methylphenyl)aminobiphenyl)benzidine), apolycarboxy-polymer, a quaternized polyamine-polymer, apolysulphato-polymer, a polysulpho-polymer, and a poly (vinylphosphonicacid).
 12. The mixed-type heterojunction thin-film solar cell structureaccording to claim 8, wherein when said inorganic wire array is a micronstructure or a submicron structure, said inorganic wire array is made ofa material selected from a group consisting of silicon, germanium,gallium arsenide, indium phosphide, gallium phosphide, antimonyselenide, indium gallium nitride, a single element, a binary compoundsemiconductor, and a compound semiconductor containing more than twocomponents.
 13. The mixed-type heterojunction thin-film solar cellstructure according to claim 8, wherein said inorganic wire array has atotal width of between 300 nm and 100 m on said substrate.
 14. Themixed-type heterojunction thin-film solar cell structure according toclaim 1, wherein said conjugate polymer layer has a thickness of between30 nm and 5 μm.
 15. The mixed-type heterojunction thin-film solar cellstructure according to claim 8, wherein each wire of said inorganic wirearray has a length of between 50 nm and 50 μm.
 16. The mixed-typeheterojunction thin-film solar cell structure according to claim 8,wherein when said inorganic wire array is a nanometric structure, asection, which is vertical to said substrate, of each wire has a widthof between 5 nm and 300 nm; when said inorganic wire array is a micronstructure, a section, which is vertical to said substrate, of each wirehas a width of between 300 nm and 3000 μm.
 17. The mixed-typeheterojunction thin-film solar cell structure according to claim 8,wherein when said inorganic wire array is a micron structure or asubmicron structure, spacing between two wires is below 100 μm; whensaid inorganic wire array is a nanometric structure, spacing between twowires is below 50 times width of a wire.
 18. The mixed-typeheterojunction thin-film solar cell structure according to claim 8,wherein when said inorganic wire array is a nanometric structure, adepth by which wires of said inorganic wire array is inserted into saidconjugate polymer layer is between 30 nm and 5 μm.
 19. A method forfabricating a mixed-type heterojunction thin-film solar cell structure,comprising steps: providing a conductive substrate and a template,wherein said template has an inorganic wire array and a substrate, andsaid inorganic wire array is formed on said substrate; forming aconjugate polymer layer on said conductive substrate; embedding saidinorganic wire array into said conjugate polymer layer, and separatingsaid substrate from said inorganic wire array; and forming an electrodelayer over said inorganic wire array and said conjugate polymer layer.20. The method for fabricating a mixed-type heterojunction thin-filmsolar cell structure according to claim 19, wherein a hole blockinglayer is formed in between said conjugate polymer layer and saidelectrode layer to cover said conjugate polymer layer and said inorganicwire array, said hole blocking layer is made of a material selected froma group consisting of ZnO (Zinc Oxide), TiO_(x) (titanium oxide), PCBM((6,6)-phenyl C₆₁ butyric acid methyl ester), LiF (Lithium Fluoride), acalcium compound and an alkali compound, wherein said alkali compound isLi₂O, LiBO₂, K₂SiO₃, or Cs₂CO₃.
 21. The method for fabricating amixed-type heterojunction thin-film solar cell structure according toclaim 19, wherein said conjugate polymer layer is heated to a glasstransition temperature thereof, and then pressure is applied to saidinorganic wire array to embed said inorganic wire array into saidconjugate polymer layer.
 22. The method for fabricating a mixed-typeheterojunction thin-film solar cell structure according to claim 19,wherein a non-conductive layer is formed in between said conjugatepolymer layer and said electrode layer to protect said inorganic wirearray; said non-conductive layer has such a thickness that saidinorganic wire array emerging from said conjugate polymer layer ishigher than said non-conductive layer; said non-conductive layer is madeof a transparent material or an opaque material; said transparentmaterial is one of following materials: PMMA (polymethylmethacrylate),SOG (Spin-On Glass), SiO₂ (silicon dioxide), and Si₃N₄ (siliconnitride).
 23. The method for fabricating a mixed-type heterojunctionthin-film solar cell structure according to claim 22, wherein saidnon-conductive layer is firstly formed over said inorganic wire arrayand said conjugate polymer layer, and then said non-conductive layer isetched to have such a thickness which is shorter than a height of saidinorganic wire array emerging from said conjugate polymer layer, andthen said electrode layer is formed over said non-conductive layer andsaid inorganic wire array.
 24. The method for fabricating a mixed-typeheterojunction thin-film solar cell structure according to claim 23,wherein said non-conductive layer is etched with a dry etching method ora wet etching method.
 25. The method for fabricating a mixed-typeheterojunction thin-film solar cell structure according to claim 22,wherein a hole blocking layer is formed in between said non-conductivelayer and said electrode layer to cover said non-conductive layer andsaid inorganic wire array, said hole blocking layer is made of amaterial selected from a group consisting of ZnO (Zinc Oxide), TiO_(x)(titanium oxide), PCBM ((6,6)-phenyl C₆₁ butyric acid methyl ester), LiF(Lithium Fluoride), a calcium compound and an alkali compound, whereinsaid alkali compound is Li₂O, LiBO₂, K₂SiO₃, or Cs₂CO₃.
 26. The methodfor fabricating a mixed-type heterojunction thin-film solar cellstructure according to claim 19, wherein said substrate is separatedfrom said inorganic wire array via ultrasonic vibration, slightknockings, chemical etching, or directly lifting off said substrate. 27.The method for fabricating a mixed-type heterojunction thin-film solarcell structure according to claim 19, wherein spacing between two wiresof said inorganic wire array is below two times diffusion length ofexcitons in polymer.
 28. The method for fabricating a mixed-typeheterojunction thin-film solar cell structure according to claim 19,wherein length of wires of said inorganic wire array is adjustedaccording to an absorption coefficient of an inorganic material of saidinorganic wire array.
 29. The method for fabricating a mixed-typeheterojunction thin-film solar cell structure according to claim 19,wherein at least one hole transport layer or at least one electronblocking layer is formed in between said conductive substrate and saidconjugate polymer; either of said hole transport layer and said electronblocking layer does not contact said inorganic wire array.
 30. Themethod for fabricating a mixed-type heterojunction thin-film solar cellstructure according to claim 19, wherein said inorganic wire array is ananometric structure, a micron structure, or a submicron structure. 31.The method for fabricating a mixed-type heterojunction thin-film solarcell structure according to claim 30, wherein when said inorganic wirearray is a micron structure or a submicron structure, nanowirestructures are formed on said micron structure or said submicronstructure to increase contact area between said conjugate polymer layerand said inorganic wire array.
 32. The method for fabricating amixed-type heterojunction thin-film solar cell structure according toclaim 19, wherein a selectively-etched layer is formed in between saidinorganic wire array and said substrate; said selectively-etched layeris etched away with a chemical etching method so that said substrate isseparated from said inorganic wire array without seriously damaging saidinorganic wire array and said substrate.
 33. The method for fabricatinga mixed-type heterojunction thin-film solar cell structure according toclaim 32, wherein a selectively-etched layer is formed in between saidsubstrate and said inorganic wire array, when said substrate isseparated from said inorganic wire array, said selectively-etched layeris completely ; or said selectively-etched layer is firstly partiallyetched away, then, said substrate is separated from said inorganic wirearray with another method.
 34. The method for fabricating a mixed-typeheterojunction thin-film solar cell structure according to claim 32,wherein said chemical etching method is a wet etching method or a dryetching method.
 35. The method for fabricating a mixed-typeheterojunction thin-film solar cell structure according to claim 19,wherein said inorganic wire array is made of a material selected from agroup consisting of silicon, germanium, gallium arsenide, indiumphosphide, gallium phosphide, gallium antimonide, zinc telluride, indiumgallium nitride, a single element, a binary compound semiconductor, anda compound semiconductor containing more than two components; saidconjugate polymer layer is made of a material selected from a groupconsisting of P3HT (poly-3-hexylthiophene), MEHPPV(poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]), PCPDTBT(poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]),OC₁C₁₀-PPV (Poly[2-(3,7-dimethyloctyloxy)-5-methoxy-p-phenylenevinylene]), MDMO-PPV(poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene]), andpolyfluorene; said conductive substrate is made of one of followingmaterials: a transparent conductive substrate, a transparent-electrodeglass substrate, a transparent-electrode plastic substrate, atransparent-electrode quartz substrate and a thin metallic plate; saidelectrode layer is made of a metal, a transparent-electrode material, ora material selected from a group consisting of ITO (Indium Tin Oxide),GITO (Gallium Indium Tin Oxide), ZITO (Zinc Indium Tin Oxide), FTO(Fluorine-doped Tin Oxide), ZnO (Zinc Oxide), AZO (Aluminum Zinc Oxide)and IZO (Indium Zinc Oxide).
 36. The method for fabricating a mixed-typeheterojunction thin-film solar cell structure according to claim 19,wherein said inorganic wire array is fabrication with a dry etchingmethod or a wet etching method; said dry etching method is one offollowing methods: plasma etching, RIE (Reactive Ion Etching), HDP(high-density plasma) etching, sputtering etching, and reactive ion beametching; said wet etching method is one of following methods: chemicalsolution etching, electrochemical etching, and photo-enhancedelectrochemical etching.
 37. The method for fabricating a mixed-typeheterojunction thin-film solar cell structure according to claim 19,wherein said inorganic wire array is fabrication with one of followingmethods: chemical vapor deposition method, molecular-beam epitaxymethod, AAO (Anodic Aluminum Oxide) method, electrochemical method,hydrothermal method, and VLS (Vapor-Liquid-Solid) method; said conjugatepolymer layer is formed with one of following methods: spin coating, dipcoating, inkjet printing, contact printing, screen printing,evaporation, sputtering, parylene coating, and electrochemicaldeposition; said electrode layer is formed with one of followingmethods: screen printing, vapor deposition, sputtering, and applyingsilver glue.
 38. The method for fabricating a mixed-type heterojunctionthin-film solar cell structure according to claim 29, wherein said holetransport layer is made of a material selected from a group consistingof PEDOT (Poly(3,4-ethylenedioxythiophene)), PEDOT:PSS(Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)), TFB:TPDSi₂(poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine]:4,4′-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl), CuPc(copper phthalocyanine), and TNATA(4,4′,4″-tris-N-naphthyl-N-phenylamino-triphenylamine); saidelectron-blocking layer is made of a material selected from a groupconsisting of TPD(N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine), BFE(poly(9,9-dioctylfluorene-co-N,N′-di(phenyl)-N,N′-di(3-carboethoxyphenyl)benzidine), NPB(4,4-bis[N-(1-naphthyl-1-)-N-phenyl-amino]-biphenyl), TPTE(N,N′-diphenyl-N,N′-bis(di(3-methylphenyl)aminobiphenyl)benzidine), apolycarboxy-polymer, a quaternized polyamine-polymer, apolysulphato-polymer, a polysulpho-polymer, and a poly (vinylphosphonicacid).
 39. The method for fabricating a mixed-type heterojunctionthin-film solar cell structure according to claim 19, wherein saidsubstrate is a semiconductor substrate.
 40. The method for fabricating amixed-type heterojunction thin-film solar cell structure according toclaim 30, wherein when said inorganic wire array is a micron structureor a submicron structure, said inorganic wire array is made of amaterial selected from a group consisting of silicon, germanium, galliumarsenide, indium phosphide, gallium phosphide, antimony selenide, indiumgallium nitride, a single element, a binary compound semiconductor, anda compound semiconductor containing more than two components.
 41. Themethod for fabricating a mixed-type heterojunction thin-film solar cellstructure according to claim 19, wherein said conductive substrate doesnot contact said inorganic wire array.